General procedure: a tetrahydrofuran solution (50 ml) of 4-chloropyridine (25.0 g, 0.22 mol) was slowly added dropwise to a solution of diisopropylammonium lithium prepared from a hexane solution of 1.6 M n-butyllithium (179 ml, 0.29 mol) with diisopropylamine (33.4 g, 0.33 mol) in tetrahydrofuran (300 ml) under the protection of argon gas, keeping the reaction temperature at -78°C. After dropwise addition, stirring was continued for 30 min. Subsequently, N,N-dimethylformamide (DMF, 19.3 g, 0.26 mol) was added and the reaction mixture was gradually warmed up to room temperature. After completion of the reaction, extraction was carried out with ethyl acetate (200 ml) and 5% aqueous ammonium chloride solution (300 ml). The organic layer was separated, dried with anhydrous magnesium sulfate and concentrated under reduced pressure to remove the solvent to give the crude product 4-chloro-3-pyridinecarboxaldehyde (27 g, 86% yield) as an oil. The product was characterized by 1H-NMR (200 MHz, CDCl3): δ 7.45 (1H, d, J = 5.0 Hz), 8.69 (1H, d, J = 5.0 Hz), 9.05 (1H, s), 10.51 (1H, s).
